1. Field of the Invention
THE present invention relates to a pneumatic tire.
2. Description of the Related Art
With respect to a pneumatic tire, there has been known a pneumatic tire where land portions partitioned by main grooves extending in the tire circumferential direction and lateral grooves which intersect with the main grooves are formed on a surface of a tread portion. The tire provided with such grooves on the tread portion generates a pattern noise from a space formed between the tire and a road surface during traveling. Particularly, an air columnar resonance sound (approximately 800 Hz to 1250 Hz) attributed to the main grooves is a main cause of such a pattern noise.
The principle on generation of such an air columnar resonance sound is as follows. That is, when a tire is brought into contact with a ground, the main groove is deformed so that a volume of the main groove changes whereby air confined in the main groove repeats compression and expansion. Due to such repetition of compression and expansion, a pumping sound, that is, an air columnar resonance sound is generated from a space in the main groove which is in contact with a ground. At this point of time, the higher a pumping pressure of air in the main groove becomes, the larger an air columnar resonance sound becomes.
To reduce such an air columnar resonance sound, it is effective to decrease a flow speed of air which passes through the space formed in the main groove which is in contact with the ground.
In view of the above, for example, JP-A-10-315711 (kokai) discloses a technique where a plurality of holes are formed in a side wall of a land portion facing a main groove along the tire circumferential direction so that friction resistance between the side wall of the land portion facing the main groove and air is increased whereby a flow speed of air which passes through the main groove is decreased thus suppressing the generated air columnar resonance sound.